Increasing ionic conductivity of LiTi2(PS4)3 by Al doping

ABSTRACT

A compound represented by the general formula Li 1+x Al x Ti 2−x (PS 4 ) 3 , wherein 0.1≤x≤0.75. The above compound has been found to have high ionic conductivity. Also, the use of the compound as a solid electrolyte, in particular in an all solid-state lithium battery.

FIELD OF INVENTION

The present invention relates to a method of increasing the ionicconductivity of lithium titanium thiophosphate LiTi₂(PS₄)₃ by Al doping.

BACKGROUND ART

The all-solid-state battery system offers the possibility of high energydensity of the battery pack. In order to realize such systems, a solidelectrolyte which exhibits high ionic conductivity is demanded.LiTi₂(PS₄)₃ is a candidate for such a solid electrolyte, and has beendescribed in Kim et al., Chem. Mater. 2008, 20, 470-474; Kim et al.,Electrochemistry Communications 10 (2008) 497-501; and Shin et al.,Journal of The Electrochemical Society, 161 (1) A154-A159 (2014).

According to the method of synthesis described in Kim et al., Chem.Mater. 2008, 20, 470-474, a stoichiometric mixture of Li₂S, TiS₂ andP₂S₅ is mixed and heated under vacuum according to the followingtemperature profile:

However, the method of preparation proposed in the literature was notoptimized with respect to ionic conductivity.

SUMMARY OF THE INVENTION

With a view to solving the above-referenced problems in the preparationof lithium thiophosphate LiTi₂(PS₄)₃, the present inventor has studieddifferent aspects of this material, and this work has led to theachievement of the present invention.

In one aspect, the present invention is thus directed to a compoundrepresented by the general formula Li_(1+x)Al_(x)Ti_(2−x)(PS₄)₃, wherein0.1≤x≤0.75.

The above compound of the present invention has been found by theinventor to have high ionic conductivity.

In another aspect, the present invention relates to a method forpreparing the compound according to the present invention, comprisingthe steps of:

(a) providing a mixture of lithium sulfide Li₂S, phosphorus sulfideP₂S₅, aluminum sulfide Al₂S₃ and titanium sulfide TiS₂;

(b) subjecting the mixture prepared in step (a) to a preliminaryreaction step through mechanical milling or melt-quenching to produce anintermediate amorphous sulfide mixture;

(c) subjecting the mixture prepared in (b) to a heat treatment step at amaximum plateau temperature of at least 350° C. and less than 500° C.

In still another aspect, the present invention relates to a use of thecompound according to the present invention as a solid electrolyte.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1(a) to 1(e) are X-ray diffraction patterns of materials obtainedin Examples 1 to 4 and Comparative Example 1 respectively.

FIG. 2 presents the relationship between the ionic conductivity and thecomposition.

DETAILED DESCRIPTION OF THE INVENTION

In order to solve the above mentioned technical problem, the presentinvention provides a compound represented by the general formulaLi_(1+x)Al_(x)Ti_(2−x)(PS₄)₃; wherein 0.1≤x≤0.75, preferably 0.3≤x≤0.6,more preferably, 0.4≤x≤0.6, and most preferably, x=0.5.

When the x value is in the above-mentioned ranges and particularlywithin the preferred ranges, the material has high ionic conductivity.

Without wishing to be bound by theory, the low resistance of LiTi₂(PS₄)₃might be due to the low content of Li inside the crystal structure. Inthe compound of the present invention, Ti is substituted aliovalently byAl (Ti⁴⁺→Li⁺+Al³⁺), which appears to result in an increase in the numberof Li without changing the crystal structure, and therefore in anincreased ionic conductivity.

Without wishing to be bound by theory, the ionic conductivity isconsidered to increase by increasing the content of Al inside of thecrystal structure (probably because the number of Li is increased as thecontent of Al is increased), and then decreases above x=0.5 in spite ofthe increased number of Li and the same crystal structure (probablybecause too high a level of Li leads to a strong Li—Li interaction).

Compounds according to the invention may be observed to have peaks inpositions of 2θ=15.08° (±0.50°), 15.28° (±0.50°), 15.92° (±0.50°), 17.5°(±0.50°), 18.24° (±0.50°), 20.30° (±0.50°), 23.44° (±0.50°), 24.48°(±0.50°), and 26.66° (±0.50°) in an X-ray diffraction measurement usinga CuKα line.

<Method of Synthesis>

The Al-doped LiTi₂(PS₄)₃ according to the present invention can beobtained, for example, by a method of synthesis comprising the steps of:

(a) providing a mixture of lithium sulfide Li₂S, phosphorus sulfideP₂S₅, aluminium sulpfde Al₂S₃ and titanium sulfide TiS₂;

(b) subjecting the mixture prepared in step (a) to a preliminaryreaction step through mechanical milling or melt-quenching to produce anintermediate amorphous sulfide mixture;

(c) subjecting the mixture prepared in step (b) to a heat treatment stepat a maximum plateau temperature of at least 350° C. and less than 500°C.

Here “maximum plateau temperature” refers to the maximum temperaturemaintained in a heating vessel as commonly used in solid statechemistry, the temperature vs. time profile typically including agradual ascent phase with a controlled rate of increase of temperaturestarting from room temperature, a chosen reaction temperature maintainedeffectively constant over a period of time (the “maximum plateautemperature”), and then a descent phase wherein the temperature isbrought back down to room temperature.

The use of the above preferred method allows one to minimize levels ofimpurity and maximize ionic conductivity.

According to the above preferred method of synthesis ofLi_(1+x)Al_(x)Ti_(2−x)(PS₄)₃ starting from the sulfides of Li, Ti, Aland P, an intermediate step is used consisting of subjecting the mixtureof starting materials to a preliminary reaction step through mechanicalmilling or melt-quenching to produce an intermediate amorphous sulfidemixture. This intermediate step may also be referred to hereinafter asan “amorphasizing step”, and provides amorphous materials derived fromLi₂S—TiS₂—Al₂S₃—P₂S₅. This amorphous material is able to be heat-treatedat over 400° C. without melting. Without the intermediate step, thespecimens are melted due to the low melting point of P₂S₅ (˜270° C.).Elemental phosphorus and sulfur also have lower melting points and willthus melt before the temperatures needed to produceLi_(1+x)Al_(x)Ti_(2−x)(PS₄)₃ product in the solid phase reaction. It isconsidered that the most important effect of the intermediate step isallowing Li₂S, TiS₂Al₂S₃ and P₂S₅ to completely mix and react with eachother—the intermediate step may, as well as ensuring intimate physicalmixing, begin the chemical transformations which will lead to the finalproduct. The intermediate step may thus act to react the low meltingmaterial of P₂S₅ with Li₂S (or at least start this reaction).

In terms of starting material mole ratios, the above preferred method ofsynthesis is appropriately carried out with as close as possible to astoichiometric ratio thereof in view of the final productLi_(1+x)Al_(x)Ti_(2−x)(PS₄)₃ product to be produced. Thus, the lithiumsulfide Li₂S, phosphorus sulfide P₂S₅, aluminium sulfide Al₂S₃ andtitanium sulfide TiS₂ starting materials are generally used in aLi₂S:P₂S₅:Al₂S₃:TiS₂ mole ratio of (1+x):3:x:(4-2x).

The amorphous material obtained through the “amorphasizing step” canadvantageously be heat-treated at the temperature of 300° C.≤T≤500° C.More generally, in heat treatment step (c), the maximum plateautemperature is appropriately not more than 475° C., and preferably atleast 375° C., more preferably at least 400° C. and at most 450° C.Further, in heat treatment step (c), the maximum plateau temperatureduring heat treatment is appropriately maintained for at least 1 hourand at most 300 hours. In terms of the speed of temperature increasegoing from room temperature up to the maximum plateau temperature,before the heat treatment step, a generally appropriate range is from0.1° C. min⁻¹ to 20° C. min⁻¹. A preferred speed is in the range of 1°C. min⁻¹ to 5° C. min⁻¹. Analogous rates of temperature decrease afterthe heat treatment step, may also be used to bring the sample back downto room temperature.

In the above preferred method of synthesis, each of the method steps(a), (b) and (c) is advantageously carried out under an inert gas, forexample, nitrogen or argon, preferably argon.

As mentioned above, the intermediate step (b), also referred to hereinas an “amorphasizing step”, the intermediate step giving rise to anintermediate amorphous sulfide mixture, may be a carried out by a“melt-quenching” procedure. In an appropriate melt-quenching step, thestarting materials are heated to a temperature higher than the meltingpoint of the final product Li_(1+x)Al_(x)Ti_(2−x)(PS₄)₃, i.e. to over700° C. However, in most cases, it is preferable for a sample to be inthe equilibrium state before quenching. Therefore, it is appropriate towait for a relatively longer period and heat up more slowly, for examplewith a heating rate of 0.05° C. min⁻¹ to 20° C. min⁻¹, with a holdingtime of appropriately 3 hours to 300 hours. To quench the moltenmixture, raised to a temperature above 700° C., a rapid cooling rate isused, appropriately between 300 to 1000 K s⁻¹, to bring the mixture toroom temperature of below. A generally appropriate method for sulfideamorphous materials is ice quenching. A heated quartz tube containingthe material to be quenched is placed in ice water.

In preferred embodiments for carrying out the above preferred method,the intermediate step (b), also referred to herein as an “amorphasizingstep”, the intermediate step giving rise to an intermediate amorphoussulfide mixture, is carried out by a mechanical milling procedure, suchas planetary ball milling, vibration milling or jet milling. Where thepreferred method of planetary ball milling is used, a generallyappropriate ball size range is chosen within the range 1 mm≤ϕ≤10 mm),the temperature range is chosen within the range 0° C.≤T≤60° C., therotation speed is chosen within the range 200 rpm≤R≤500 rpm, and theduration is chosen within the range 5 h≤t≤200 h.

<Treatment>

The above preferred method of synthesis can further comprise thefollowing steps of treatment, comprising the steps of:

(d) compressing the lithium aluminium titanium thiophosphate sampleprovided in step (c) to form a compressed powder layer; and

(e) sintering the lithium aluminium titanium thiophosphate obtained as acompressed powder layer in step (d) at a temperature of at least 200° C.and at most 400° C.

The above preferred method of treatment has been found by the inventorto enable higher ionic conductivity to be obtained for this type ofmaterial.

From the results obtained by the present inventor, the sinteringtemperature T range of 200° C.≤T≤400° C. is observed to be preferable toobtain high ionic conductivity of Li_(1+x)Al_(x)Ti_(2−x)(PS₄)₃. A morepreferred range is a temperature of at least 250° C. and at most 375° C.Particularly preferred sintering temperature ranges are from at least275° C. to at most 375° C.

In the above preferred method of treatment, in step (d), the sample oflithium titanium thiophosphate may preferably be compressed at apressure range of at least 100 MPa and at most 1500 MPa. The compressionstep (d) may give rise to samples in pellet form, but the size of thepellets is not important in producing the final effect of increasedionic conductivity.

In the above preferred method of treatment, in step (e), the sinteringtime is at least 1 hour and at most 300 hours. Here, by “sintering time”reference is made to the plateau temperature maintained in a heatingprogramme. Concerning the temperature vs. time heating profileappropriate before and after reaching the sintering plateau temperature,a generally appropriate heating rate is 0.1° C. min⁻¹ to 20° C. min⁻¹.After the sintering time appropriately of 1 hour to 300 hours at thesintering temperature of of 200° C.≤T≤400° C., cooling can be carriedout by natural cooling or controlled cooling. If the cooling process iscontrolled, the cooling rate may appropriately be 0.1° C. min⁻¹ to 100°C. min⁻¹.

<All Solid-State Lithium Battery>

In a further aspect, the present disclosure relates to anall-solid-state lithium battery comprising the following elements:

a positive electrode active material layer;

a solid electrolyte layer;

a negative electrode active material layer,

wherein the solid electrolyte layer contains an Al-doped lithiumtitanium thiophosphate LiTi₂(PS₄)₃ material produced according to thepresent invention, and is positioned between the positive electrodeactive material layer and negative electrode active material layer.

In such an all-solid-state lithium battery, using as solid electrolyte,the Li_(1+x)Al_(x)Ti_(2−x)(PS₄)₃ sulfide materials according to thepresent invention, concerning the form of the solid electrolytematerials, examples include a particle shape, such as the shape of atrue ball and the shape of an elliptical ball, or a thin film form, forexample. When solid electrolyte materials have a particle shape, as forthe mean particle diameter, it is preferable that their size is withinthe range of 50 nm to 10 micrometers, more preferably within the rangeof 100 nm to 5 micrometers.

Although it is preferable to have only one or more solid electrolytematerials as mentioned above in a solid electrolyte layer, this layermay also contain a binding agent if needed. As a binding agent used fora solid electrolyte layer, this may be of the same type as mentionedhereunder for the positive active material layer.

As regards the thickness of a solid electrolyte layer, although this maychange with the kind of solid electrolyte materials, and the overallcomposition of an all-solid battery, generally it is preferable thatthis thickness is within the range of 0.1 micrometer to 1000micrometers, more preferably within the range of 0.1 micrometer to 300micrometers.

Concerning the positive active material (cathode active material) to beused in the positive electrode (cathode) active material layer, this isnot especially limited if the average operating potential becomes morethan 4 V (vs. Li/Li⁺). As an average operating potential of positiveactive material, this is appropriately more than 4 V (vs. Li/Li⁺), andit is preferable that it is within the limits of 4.0 V to 6.0 V, stillmore preferably within the limits of 4.5 V to 5.5 V. The averageoperating potential can be evaluated using cyclic voltammetry, forexample. In particular, when cyclic voltammetry is measured at a smallelectric potential speed like 0.1 mV/sec, it can be considered that theaverage value of the voltage which gives the peak current on the side ofoxidation, and the voltage which gives the peak current on the side ofreduction is the average operating potential.

As a positive active material, especially if the average operatingpotential is made with more than 4 V (vs. Li/Li⁺), there is no specificlimitation, but it is preferable that the material is an oxide positiveactive material, which can have a high energy density.

A compound which has the spinel type structure denoted by generalformula LiM₂O₄ (M is at least one kind of transition metal element), asan example of positive active material, can be mentioned as an example.As regards M of the above-mentioned general formula LiM₂O₄, especiallyif it is a transition metal element, it will not be limited, but it ispreferable that it is at least one kind chosen from the group whichconsists of Ni, Mn, Cr, Co, V, and Ti, for example, and it is morepreferable that it is at least one kind chosen from the group whichconsists of Ni, Mn, and Cr especially. Specifically,LiCr_(0.05)Ni_(0.50)Mn_(1.45)O₄, LiCrMnO₄, LiNi_(0.5)Mn_(1.5)O₄, etc.can be mentioned. The compound which has the olivine type structuredenoted by general formula LiMPO₄ (M is at least one kind of transitionmetal element) as other examples of positive active material can bementioned. M in the above-mentioned general formula will not be limitedespecially if it is a transition metal element, but it is preferablethat it is at least one kind chosen from Mn, Co, Ni, and the group thatconsists of V, for example, and it is more preferable that it is atleast one kind chosen from the group which consists of Mn, Co, and Niespecially. Specifically, LiMnPO₄, LiCoPO₄, LiNiPO₄, etc. can bementioned. The compound which has the layer structure denoted by generalformula LiMO₂ (M is at least 1 type of a transition metal element) asother examples of positive active material can be mentioned.Specifically, LiCoO₂, LiNi_(0.5)Mn_(0.5)O₂ andLiNi_(0.33)Co_(0.33)Mn_(0.33)O₂ etc. can be mentioned. As examples otherthan the positive active material mentioned above, aLi₂MnO₃—LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂ solid solution, aLi₂MnO₃—LiNi_(0.5)Mn_(1.5)O₂ solid solution, a Li₂MnO₃—LiFeO₂ solidsolution, etc. can be mentioned.

As regards the form of the positive active material, a particle shape,such as the shape of a true ball and the shape of an elliptical ball,thin film form, etc. can be mentioned, as an example. As for the meanparticle diameter, when the positive active material has a particleshape, it is preferable that it is within the size range of 0.1micrometer to 50 micrometers, for example. As for the content of thepositive active material in a positive active material layer, it ispreferable that it is in the range of 10% by weight to 99% by weight,for example, more preferably from 20% by weight to 90% by weight.

Concerning the positive active material layer, in addition to thepositive active material mentioned above, if needed, the positive activematerial layer in may contain other materials, for example, solidelectrolyte materials etc. As for the content of the solid electrolytematerials in a positive active material layer, it is preferable thatthis content is 1% by weight to 90% by weight, more preferably 10% byweight to 80% by weight.

Furthermore, a positive active material layer may contain anelectrically conductive agent from a viewpoint of improving theconductivity of a positive active material layer, other than the solidelectrolyte materials mentioned above. As electrically conductivematerial, acetylene black, Ketjenblack, a carbon fiber, etc. can bementioned, for example. A positive active material may also contain abinding agent. As such a binding material (binding agent),fluorine-based binding materials, such as polyvinylidene fluoride (PVDF)and polytetrafluoroethylene (PTFE), etc. can be mentioned, for example.

Although the thickness of a positive active material layer may changeaccording to the kind of all-solid-state battery made, it is generallypreferable that it is within the range of 0.1 micrometer to 1000micrometers.

As regards the negative electrode active material layer, this layer atleast contains one or more negative electrode active material(s), andmay additionally contain at least one or more of solid electrolytematerials and electrically conductive agents if needed. Forall-solid-state lithium batteries, the negative electrode activematerial is not limited provided that occlusion and discharge of the Liion, which is a conduction ion, are possible. As a negative electrodeactive material, a carbon active material, a metal active material, etc.can be mentioned, for example. As a carbon active material, black lead,meso carbon micro beads (MCMB), highly ordered/oriented pyrolyticgraphite (HOPG), hard carbon, soft carbon, etc. can be mentioned asexamples. On the other hand, as a metal active material, charges of analloy, such as Li alloy and Sn—Co—C, In, Al, Si, Sn, etc. can bementioned as examples. Oxide stock materials, such as Li₄Ti₅O₁₂, can bementioned as examples of other negative electrode active materials.

Concerning solid electrolyte materials used for the negative electrodeactive material layer, and an electrically conductive agent, these maybe the same as that for the solid electrolyte layer and positive activematerial layer mentioned above.

The thickness of the negative electrode active material layer willgenerally be appropriately within the range of 0.1 micrometer to 1000micrometers.

An all-solid-state battery of the present disclosure has at least thepositive active material layer, solid electrolyte layer, and negativeelectrode active material layer which were mentioned above. It furtherusually has a positive pole collector which collects a positive activematerial layer, and a negative pole collector which performs currentcollection of a negative electrode active material layer. As a materialof a positive pole collector, for example, SUS (stainless steel),aluminum, nickel, iron, titanium, carbon, etc. can be mentioned, and SUSis especially preferable. On the other hand as a material of a negativepole collector, SUS, copper, nickel, carbon, etc. can be mentioned, forexample, and SUS is especially preferable. Concerning the thickness,form, etc. of a positive pole collector and a negative pole collector,the person skilled in the art may choose suitably according to the useof the all-solid-state battery, etc. The cell case used for a commonall-solid-state battery can be used, for example, the cell case madefrom SUS, etc. can be mentioned. The all-solid-state battery may form apower generation element in the inside of an insulating ring.

The all-solid-state battery of the present disclosure can be consideredas a chargeable and dischargeable all-solid-state battery in a roomtemperature environment. Although it may be a primary battery and may bea rechargeable battery, it is especially preferable that it is arechargeable battery. Concerning the form of the all-solid-statebattery, a coin type, a laminated type, cylindrical, a square shape,etc. can be mentioned, as examples.

As regards the manufacturing method of the all-solid-state battery, thisis not particularly limited, and common manufacturing methods ofall-solid-state batteries can be used. For example, when anall-solid-state battery is in the thin film form, a positive activematerial layer can be formed on a substrate, and the method of forming asolid electrolyte layer and a negative electrode active material layerin order, and laminating them thereafter etc., may be used.

Within the practice of the present invention, it may be envisaged tocombine any features or embodiments which have hereinabove beenseparately set out and indicated to be advantageous, preferable,appropriate or otherwise generally applicable in the practice of theinvention. The present description should be considered to include allsuch combinations of features or embodiments described herein unlesssuch combinations are said herein to be mutually exclusive or areclearly understood in context to be mutually exclusive.

EXAMPLES

The following experimental section illustrates experimentally thepractice of the present invention, but the scope of the invention is notto be considered to be limited to the specific examples that follow.

Example 1: x=0.1

Synthesis of Solid Electrolytes

Mixture Step

The solid electrolyte Li_(1+x)Al_(x)Ti_(2−x)(PS₄)₃ was synthesized usingstarting materials Li₂S (Sigma), TiS₂ (Sigma), Al₂S₃ (Sigma) and P₂S₅(Aldrich). They were mixed at the weight ratio listed in Table 1 below.

Amorphasizing Step

The mixed sample was put into the zirconium pot (45 mL) with 18zirconium balls (Ø10 mm) under Argon. The pot was closed and treatedwith planetary milling equipment (Fritsch, P7) at 370 rpm for 40 h toobtain the precursor.

Heat Treatment Step

The precursor was sealed into the glass tube at the pressure of 30 Paand then heated at T=400° C. for 8 h.

Measurement of Li Ion Conductance

Li ion conductance at a temperature of 25° C. was measured using thesulfide solid electrolyte material obtained. First, 100 mg of thesulfide solid electrolyte material was added to a cylinder made ofalumina and pressed at 4 ton/cm² to form a solid electrolyte layer. Thepellet was sandwiched by SUS current collector for measuring impedancespectroscopy.

An impedance gain-phase analyzer manufactured by Biologic (VMP3) wasused for the measurement as FRA (Frequency Response Analyzer). Themeasurement was started from a high-frequency range on the conditions ofan alternating voltage of 5 mV, a frequency range of 1 Hz to 1 MHz.

The ionic conductivity of Example 1 was 4.2×10⁻⁵ S/cm.

X-Ray Diffraction Measurement

X-ray diffraction measurement (using a CuKα line) was performed by usingthe sulfide solid electrolyte materials obtained in the Examples and inthe Comparative Example. The results are shown in FIG. 1.

For samples prepared according to the Examples, and essentially only thepeaks of the type observed for LiTi₂(PS₄)₃ were detected.

Example 2: x=0.25

Only the x value was different from Example 1—here, the x value was0.25. The ionic conductivity of the material produced by Example 2 was4.3×10⁻⁵ S/cm. Essentially, only the peaks of the type observed forLiTi₂(PS₄)₃ were detected by XRD.

Example 3: x=0.5

Only the x value was different from Example 1—here, the x value was 0.5.The ionic conductivity of the material produced by Example 3 was7.1×10⁻⁵ S/cm. Essentially, only the peaks of the type observed forLiTi₂(PS₄)₃ were detected by XRD.

Example 4: x=0.75

Only the x value was different from Example 1—here, the x value was0.75. The ionic conductivity of the material produced by Example 4 was2.9×10⁻⁵ S/cm. Essentially, only the peaks of the type observed forLiTi₂(PS₄)₃ were detected by XRD.

Comparative Example 1: x=0

Only the x value was different from Example 1—here, the x value was 0.The ionic conductivity of the material produced by Comparative Examplewas 2.5×10⁻⁵ S/cm. Essentially, only the peaks of the type observed forLiTi₂(PS₄)₃ were detected by XRD.

Composition Optimization

From the results of Examples and Comparative Examples presented in FIG.2, it can be noted that it is possible to obtain a high ionicconductivity of Li_(1+x)Al_(x)Ti_(2−x)(PS₄)₃ in the range of 0.1≤x≤0.75.

As shown by Examples 1, 2 and 3, the ionic conductivity was increased byincreasing the content of Al inside of the crystal structure. This isthought to be probably because the number of Li was increased as thecontent of Al was increased.

As shown by Examples 3 and 4, the conductivity was decreased above x=0.5in spite of the increased number of Li and the same crystal structure.This is thought to be probably because too high a level of Li leads to astrong Li—Li interaction.

TABLE 1 weight of starting materials (g) Comparative Example 1 Example 2Example 3 Example 4 Example 1 x 0.1 0.25 0.5 0.75 0 Li₂S 0.0436 0.04980.0601 0.0705 0.0396 P₂S₅ 0.5759 0.5779 0.5815 0.5850 0.5745 TiS₂ 0.36750.3397 0.2930 0.2456 0.3859 Al₂S₃ 0.0130 0.0325 0.0655 0.0988 0.0000

The invention claimed is:
 1. A solid electrolyte compound represented bythe general formula Li_(1+x)Al_(x)Ti_(2−x)(PS₄)₃, wherein 0.1≤x≤0.75. 2.The solid electrolyte compound according to claim 1, wherein 0.3≤x≤0.6.3. The solid electrolyte compound according to claim 1, wherein0.4≤x≤0.6.
 4. The solid electrolyte compound according to claim 1,wherein x=0.5.
 5. The solid electrolyte compound according to claim 1,having peaks in positions of 2θ=15.08° (±0.50°), 15.28° (±0.50°), 15.92°(±0.50°), 17.5° (±0.50°), 18.24° (±0.50°), 20.30° (±0.50°), 23.44°(±0.50°), 24.48° (±0.50°), and 26.66° (±0.50°) in an X-ray diffractionmeasurement using a CuKα line.
 6. Method for preparing the solidelectrolyte compound according to claim 1, comprising the steps of: (a)providing a mixture of lithium sulfide Li₂S, phosphorus sulfide P₂S₅,aluminum sulfide Al₂S₃ and titanium sulfide TiS₂; (b) subjecting themixture prepared in step (a) to a preliminary reaction step throughmechanical milling or melt-quenching to produce an intermediateamorphous sulfide mixture; (c) subjecting the mixture prepared in (b) toa heat treatment step at a maximum plateau temperature of at least 350°C. and less than 500° C.
 7. Method according to claim 6, furthercomprising the following steps of surface treatment: (d) compressing thesolid electrolyte compound provided in step (c) to form a compressedpowder layer; and (e) sintering the solid electrolyte compound obtainedas a compressed powder layer in step (d) at a temperature of at least200° C. and at most 400° C.
 8. A method comprising including the solidelectrolyte compound according to claim 1 as a solid electrolyte in anall-solid-state battery system.
 9. A method comprising including thesolid electrolyte compound according to claim 1 as a solid electrolytein an all solid-state lithium battery.
 10. Solid-state batterycomprising the solid electrolyte compound according to claim 1 as asolid electrolyte.
 11. All-solid state lithium battery comprising thefollowing elements: a positive electrode active material layer; a solidelectrolyte layer; a negative electrode active material layer, whereinthe solid electrolyte layer contains the solid electrolyte compoundaccording to claim 1.